Pranayam the fourth limb of Ashtang Yoga is generally known as control of breath. Pranayam word has two parts:- Prana & AYAM. Prana means vital energy the life forcewhich exists in all things. 'YAM' Means control & 'AYAM' means extension or expansion. So, the word Pranayam means extension or expansion of the dimension of Prana the life force. By Practicing'Pranayam' one can get extra oxygen into the lungs, attaining a higher state of vibratory energy. Swami Vivekanada has termed Pranayam as Sanjeevani Shakti (the psychic force).
A sadhak (Devotee) can attain the perfection (Sidhi) of many types by Pranayam. By practicing Pranayam, one can gain health at physical, mental & spiritual level. Flow of Prana In the nadis, purifies, regulates & activates them, inducing physical & mental stability. It is an invisible power which can be felt and can not be seen.
The breath is the most vital process of the body. Human beings breathe about 15 times per minute & 21,600 times per day. Most people breathe incorrectly, using only a small part of their lung capacity. Pranayam regulates breathing process, resulting in natural, relaxed rhythms of the body & mind. Although breathing is mainly an unconscious process, conscious control of it forms a bridge between the conscious & unconscious areas of the mind. The ancients yogis & sages studied nature in great detail. They noticed that animals with a slow breath rate such as pythons, elephants & tortoises have long life spans, whereas those with a fast breathing rate, such as birds, dogs & rabbits live for only a few years. From this observation they realized the importance of slow breathing for increasing the human life span. This is because the respiration is directly related to the heart. A slow breathing rate keeps the heart stronger, contributing to a longer life. Deep breathing also increases the absorption of energy by the 'Pranamaya kosh', enhancing Dynamism vitality & general well being. By deep breathe one can control the breathing upto 7200 times a day. Remember there is an intimate connection between the breath, nerve currents & control of the inner Prana or vital forces. On physical Prana becomes visible as motion & action on the mental plane as thought.

PRANAYAMS
PARANYAMS are of many types -
1. NADI SHUDDHI PRANAYAM
2. ANULOM VILOM (Opposite)
3. SHEETALI
4. SEETKARI
5. BHRAMARI
6. BHASTRKA
7. KAPAAL BHATI
8. UJJA YEE
9. MOKSHA PRANAYAM

There are three steps of PRANAYAM
RECHAK- Exhaling breath
POORAK- Inhaling breath, and
KUMBHAK Retaining the breath.
Inhaling the breath in and holding up is inner KUMBHAK and holding up after exhaling breath is called outer KUMBHAK.
Nose has two sounds -
CHANDRABHEDI SWAR and SURYABHEDI SWAR.

The left part of the nose is CHANDRA (The Moon) SWAR and the right part is SURYA (The Sun) SWAR.